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    <title>Document</title>
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<body>
    <div>
        <h2>promise 封装 ajax</h2>
        <button id="btn">点击发送ajax</button>
    </div>

    <script>
        let btn = document.querySelector("#btn");

        btn.addEventListener("click",function(){
            const xhr = new XMLHttpRequest();
            xhr.open("get","https://api.apiopen.top/getJoke");
            xhr.send();
            xhr.onreadystatechange = function(){
                if(xhr.readyState === 4){
                    if(xhr.status>= 200 && xhe.status<300){
                        console.log(xhr.response);
                    }else{
                        console.log(xhr.status);
                    }
                }
            }
        })
    </script>
</body>
</html>